I wonder how hard it would be to remove that compartment. It looks like it was siliconed to the walls and floor. It would free up an extra 3 or 4 gallons of swimming space. But might be more bother than it is worth.
If you remove compartment, (and don't block the hole on the bottom) during a power outage or some other unforeseen event, all the water in the tank could completely drain out thru that hole. Gravity rules in this case.
And as said before, the weir of the compartment determines the surface, of the water in the tank, and holds it back from all leaving the tank at once.
For a tank such as this, IMO using a sump is the most advantageous filtration type, way to go.
Where I live, earthquakes are common, and have loosened up the seems of my 180 gal tank, creating constant leaks.
To take a little outward pressure off, I have just drilled a new hole, about 4-5" from the rim.
The tank also being used, was predrilled with 2 holes (below), about 2" from the rim.
Below is the new hole, drilled lower, that may take enough outward pressure off the walls to slow the constant seepage
I run a 125 gal sump, as filtration for the 180.
The sump is below.
Water drains out of the tank, by gravity, thru the (now) 3 holes.
The depth they are placed, determines the depth of the main tank.
There is a pump, in the sump that returns water back to the tank, in a continuous circle.
The pump is rated at 1500 gph,
You could place a sump in the cabinet, ......off to the side,....... even on a floor below.
The only constant or rule, is the law of gravity
Below is what I mean by, having the sump (right) off to the side of the tank.
